One hundred forty-nine patients who underwent surgery for tubal pregnancy at five hospitals in Seattle (WA) between 1975 and 1979 were interviewed to determine the risk factors for this disorder. Their responses were compared with those of 706 control women who had conceived an intrauterine pregnancy during the same time period during which the tubal pregnancies occurred. A higher proportion of cases reported a history of infertility (attempt to conceive without success for a period of at least 1 year) than controls (relative risk [RR] = 2.5; 95% confidence interval [CI] = 1.7-3.7). Women who were diagnosed in the investigation of their infertility as having tubal or ovulatory dysfunction had relative risks of tubal pregnancy of 5.8 (95% CI = 2.1-16.4) and 3.4 (95% CI = 1.3-8.5), respectively. The average time over which subjects had attempted to conceive before index pregnancies that were planned was longer among cases (15.4 months) than among controls (6.9 months). These results support the hypothesis that a history of infertility predisposes women to an increased risk of tubal pregnancy. The authors also found that, among infertile women, about twice as many cases (14.3%) as controls (6.8%) were current fertility drug users (RR = 3.1; 95% CI = 1.1-9.1), which suggests that the use of fertility drug(s) may further increase the risk of tubal pregnancy.